Add RESTful method support for html forms

This commit allows calls to `CHtml::form()`, `CHtml::beginForm`
`CActiveForm::$method` etc to accept verbs other than `post` and `get`,
e.g. `delete`, `put`, `patch` etc

This is implemented by detecting non `post`, `get`, converting to `post`
 and adding a hidden `_method` field, similar to how CSRF protection works.
update changelog
add absolutePaths option to CFileHelper::findFiles

if absolutePaths is true (by default), function will return absolute paths, otherwise the relative ones
mbstring fallback for CFileHelper::findFiles
Double condition applying for Active Record relation lazy loading has been fixed.
Issue #2756 has been added to CHANGELOG
code conv fix, charset specifying and docs improvement
Merge remote-tracking branch 'upstream/master' into patch-1
CHANGELOG updated
another way to return relative paths: findFilesRecursive main loop instead of findFiles post-processing
CClientScriptTest has been advanced to expose issue #2770
CClientScript::renderScriptBatch() has been fixed to group scripts with different HTML options correctly.
Issue #2770 has been added to CHANGELOG
CHANGELOG for issue #2770 has been updated.
Merge pull request #2761 from klimov-paul/2756-relation-lazy-load-with-condition-doubles-condition

Fix for #2756: relation lazy load with condition doubles condition
allow arrays in asc/desc properties of virtual attributes
updated CHANGELOG and added test
added unit test to verify behavior for #2756
CClientScript::renderScriptBatch() has been improved to sort html options only if they are specified.
Merge branch 'master' of github.com:yiisoft/yii into 2770-client-script-render-script-batch